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Science: 'Unstable' molecules find their feet

Chemists have found that several molecules that were thought to have only a fleeting existence are stable. The discoveries overturn some long-held theories about molecular stability.

A team at the American company Du Pont has made a compound known as a carbene, and even grown crystals of it. Another team at the University of Chicago has made simple versions of compounds known as enols, and found that they can survive for several weeks at room temperature.

Steven Bergens and Brice Bosnich at Chicago discovered the stable enols-alcohols with a double bond next to the alcohol group. In doing so, they have opened up a whole new area of organic chemistry.

Until now, chemists have assumed that all enols are unstable because the hydrogen atom readily moves to make either an aldehyde or a ketone.

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Bergens and Bosnich made enols by moving a double bond so that it was next to an alcohol group. They expected the enols to respond to the double bond by immediately converting to aldehydes or ketones. To their surprise, the enols were quite stable. If the reaction is carried out in a dry solvent, and acids and bases which catalyse the rearrangement are kept safely away, the enols are stable for days, say the chemists (Journal of the American Chemical Society, vol 113, p 958).

Now it will be possible to study simple enols, with the aid of techniques such as nuclear magnetic resonance spectroscopy (NMR). Bergens and Bosnich have already made eight simple enols which are stable. Each can exist in two forms, which differ in the way the OH group is placed with respect to other groups of atoms attached to the double bond. Bergens and Bosnich were able to identify them with NMR. They found that both forms appear in roughly equal amounts.

The chemists’ secret for making enols is a complex rhodium compound, in which the rhodium atom is gripped by claw-like molecules. The rhodium compound catalyses the transfer of a hydrogen atom from the carbon atom next to the OH to a carbon that is two atoms removed. To compensate, the molecule has to move its double bond to the enol position. The reactions take about 9 minutes to form 1-methyl-propenol and 50 hours to form 2-ethyoxy-propenol. The yield of enol can be more than 90 per cent.

Although reasonably stable, the new enols are still less stable than the corresponding aldehydes and ketones. In time, the enols re-converted into these counterparts. However, the process is slower than chemists would have predicted. Bergens and Bosnich say they could still detect propenol and 1-methyl-propenol in solution that had been standing two weeks in the laboratory.

The discovery opens the door to studying enols in ways that until now have not been possible. Bergens and Bosnich have already carried out some reactions that demonstrate that the enols behave as theory predicts.

Other chemists have made another kind of notoriously unstable molecule, called a carbene, and found it to be stable. Anthony Arduengo, Richard Harlow and Michael Cline of Du Pont actually designed their carbene so it would be stable. A carbene is a molecule containing a carbon atom with just two single bonds. Carbon usually has four bonds, which explains why chemists consider carbenes to be very unstable compounds.

The new compound is so stable that the chemists have been able to grow crystals of it and subject them to X-ray analysis to prove it is a carbene. They have also used other methods of analysis such as NMR, infrared spectroscopy and mass spectrometry, to show that the molecule is indeed a carbene.

Normally, carbon forms four bonds with its four outer electrons, making either four single bonds, or two single bonds and one double bond, or two double bonds, and so on. In a carbene, carbon forms two single bonds, and the remaining two electrons are a lone pair. Carbenes are known to have a fleeting existence as intermediates in chemical reactions, but chemists would not expect such intermediates to be stable.

Arduengo and his colleagues have made the first stable carbene – stable, that is, in the absence of air and moisture (Journal of the American Chemical Society, vol 113, p 361). It melts at a temperature of 24 °C, and it can be melted and remelted without decomposing.

The compound owes its stability to two large adamantyl groups, which are placed in such a way that they prevent other molecules from gaining access to the carbene carbon once it has been formed. This key atom is placed between two nitrogen atoms, each of which carries an adamantyl group.

The chemists made the carbene using sodium hydride (NaH) with dimethyl sulphoxide, in the solvent tetrahydrofuran. This reagent plucks off the hydrogen from the carbon and forms the carbene.